Impact of non-reciprocal channel conditions in broadband TDD systems

Multi-path fading gives rise to frequency-dependent channel conditions. For narrowband TDD systems, channel conditions in uplink and downlink (or forward and reverse) can be assumed reciprocal. For broadband TDD systems applying frequency domain scheduling, reciprocal channel conditions are no longer valid. Using the coherence bandwidth for assessing the correlation between uplink and downlink can lead to invalid conclusions. By using cumulative distribution functions derived from a large number of channel realizations, it is shown that the radio attenuation at relatively small frequency offsets can differ by several dBs for a large percentage of the cases. This is particularly a problem in cognitive (UWB) radios that apply detect-and-avoid techniques and rely on uplink detection to protect the downlink reception. Until now, non-reciprocity between uplink and downlink channels was considered a challenge in case of detecting FDD systems. However, in this paper it is shown that even for detecting broadband TDD systems, reciprocity between uplink and downlink channels can no longer be assumed. Other challenges arise when adaptive antennas are used without feedback and downlink parameters are based on uplink measurements.